Most motor vehicles which are propelled by an internal combustion engine typically include a fuel tank. Replenishment of fuel in the tank is usually achieved by inserting a pressure-sensitive nozzle which is in fluid communication with a filler pipe and a supply of fuel. Upon insertion of the pressure-sensitive nozzle into a fuel filler neck attached to the fuel tank, fuel is delivered from the filler pipe until fuel flow is arrested by an operator or by the fuel in the tank reaching a shut-off level.
After depletion of fuel during consumption by the internal combustion engine, the level of fuel rises as the tank is filled from the supply. As is well known, the space in the fuel tank above the surface of the liquid fuel becomes occupied by an amount of fuel vapor. To avoid unwanted build-up of fuel vapor pressure, a vent tube is typically provided which ducts fuel vapor from the fuel tank ultimately to the ambient atmosphere.
A fuel shut-off level is reached when the rising surface of fuel in the fuel tank is higher than the open end of the vent tube which extends into the fuel tank. Before reaching the fuel cut-off level, the amount of vapor pressure approximates ambient atmospheric pressure because of unobstructed relief through the vent tube. Beyond the fuel shut-off level, if more fuel were delivered into the fuel tank from the supply through the pressure-sensitive fuel filler pipe nozzle, the vapor pressure in the space above the fuel in the tank would rise, because the fuel vapor has no means of escape through the vent tube. In response to the build-up of fuel vapor pressure, the pressure-sensitive fuel filler pipe nozzle terminates the delivery of fuel into the tank.
One difficulty in implementing the aforesaid general design objectives lies in the unpredictable effect on fuel level of dimensional and shape changes in the fuel tank caused by temperature variations and mechanical deformation. The materials of which fuel tanks are made inherently possess coefficients of thermal expansion which produce dimensional changes in response to variations in ambient temperature. To meet crashworthiness standards and weight requirements imposed by regulatory authorities and the vehicle designer, contemporary fuel tanks are typically made of deformable, but thin materials. Consequently, dimensional changes in the fuel tank also result when the tank is affixed to the vehicle underbody by retaining straps. Such straps generally require tensioning in order to provide firm securement of the fuel tank to the vehicle underbody. Because of lack of rigidity in the fuel tank, the effect of such tensioning is to cause some buckling of the fuel tank walls.
One adverse result of such thermally and mechanically induced dimensional changes is that the location of the fuel cut-off level is altered. In extreme cases, the fuel cut-off level, defined by the highest point of the open end of the vent tube within the fuel tank, may be displaced downwardly toward the lower portion of the fuel tank if buckling occurs in the tank wall which supports the vent tube. As a result, the amount of useable fuel which an operator may add to the tank may be diminished significantly. In such cases, the amount of useable fuel in the fuel tank may be considerably below the tank's nominal capacity.
Thus, a causative factor involved in sub-optimal fuel replenishment is the linear or angular displacement of the fuel shut-off tube in response to thermally or mechanically-induced dimensional changes in the fuel tank. One manifestation of such displacement is that the amount of fuel in the tank which has risen to the fuel shut-off level is unpredictable. Not only may the operator be unable to fill the fuel tank to its nominal capacity, but the amount of fuel capable of being delivered to the engine, may often be unpredictable.
A system for controlling discharge of fuel vapor from the fuel tank is disclosed in U.S. Pat. No. 4,790,349. However, this reference does not confront the problems posed by unpredictable fuel shut-off levels.